Sensory system evolution at the origin of craniates

The multiple events at the transition from non-craniate invertebrate ancest ors to craniates included the gain and/or elaboration of migratory neural c rest and neurogenic placodes. These tissues give rise to the peripherally l ocated, bipolar neurons of all non-visual sensory systems. The brain was al so elaborated at or about this same time. Were the peripheral and central e vents simultaneous or sequential? A serial transformation hypothesis postul ates that paired eyes and an enlarged brain evolved before the elaboration of migratory neural crest-placodal sensory systems. Circumstantial evidence for this scenario is derived fi um the independent occurrence of the combi nation of large, paired eyes plus a large, elaborated brain in at least thr ee taxa (cephalochordates, arthropods and craniates) and partly from the ex clusivity. of the diencephalon for visual system-related distal sensory com ponents versus the restricted distribution of migratory neural crest-placod al sensory systems to the remaining parts of the neuraxis. This scenario ac counts for the similarity of all central sensory system pathways due to the primary establishment of descending visual pathways via the diencephalon a nd midbrain tectum to brainstem motor regions and the subsequent exploitati on of the same central beachhead by the migratory neural crest-placodal sys tems as a template for their organization.